In recent years, plant growth using artificial light sources has been the subject of much research. In particular, cultivation methods using illumination from light emitting diodes (abbreviation: LED), which exhibit excellent monochromaticity, provide favorable energy conservation and long life, and can be easily miniaturized, are garnering much attention.
Based on the results of research to date, one emission wavelength band that has been confirmed as an effective light source for plant growth (photosynthesis) is red light having a wavelength within the region from 600 to 700 nm.
Light within the wavelength vicinity of 660 to 670 nm exhibits particularly favorable reaction efficacy for photosynthesis, and is thus a preferred light source. In order to generate this wavelength, light emitting layers composed of AlGaAs or InGaNP or the like are currently being investigated (for example, see Patent Documents 1 to 3).
On the other hand, compound semiconductor LEDs having a light emitting layer composed of an aluminum-gallium-indium phosphide (composition formula: (AlXGa1-X)YIn1-YP, wherein 0≦X≦1 and 0<Y≦1) are also known.
Among these types of LED compound semiconductors, a light emitting layer having the composition Ga0.5In0.5P exhibits the longest wavelength, and the peak wavelength obtained from this light emitting layer is in the vicinity of 650 nm.
Further, generally, a light emitting unit having a light emitting layer composed of (AlXGa1-X)YIn1-YP (wherein 0≦X≦1 and 0<Y≦1) is formed on a monocrystalline substrate of gallium arsenide (GaAs), which blocks the light from the light emitting layer and is not particularly strong mechanically.
Accordingly, much research is being conducted with the aims of obtaining higher brightness visible light LEDs, and achieving further improvements in the mechanical strength of these devices.
For example, Patent Document 4 discloses a so-called bonded LED prepared by removing the substrate material such as GaAs that blocks the light from the light emitting layer, and then bonding a support layer which is formed from a material that can transmit the light from the light emitting layer and exhibits excellent mechanical strength.
Patent Document 5 describes investigations that have been conducted, for laser devices having a different light emission mechanism, into light emitting layers having strain (hereafter also referred to as “strained light emitting layers”). However, there are currently no practical applications of strained light emitting layers in the field of light emitting diodes.
Patent Document 6 discloses the application of a quantum well structure to the light emitting unit of a light emitting diode. However, because the quantum effect obtained by utilizing a quantum well structure shortens the emission wavelength, this technique cannot be applied to techniques requiring wavelength lengthening.